Method for determining the completeness of cathodic protection of corrodible metal structure

ABSTRACT

A METHOD FOR EVALUATING THE EFFECTIVENESS OF A CATHODIC PROTECTION SYSTEM IN WHICH THE COMPLETENESS OF CATHODIC PROTECTION OF A CORRODIBLE METAL STRUCTURE EXPOSED TO A CORROSIVE ENVIRONMENT IS ASCERTAINED BY MEASURING THE POLARITY OF THE STRUCTURE WITH RESPECT TO A SPECIMEN OF THE SAME METAL PLACED IN THE CORROSIVE ENVIRONMENT AND ELECTRICALLY CONNECTED TO THE STRUCTURE AND MAINTAINED IN ELECTROLYTIC CONTACT WITH THE CORROSIVE ENVIRONMENT AND OUT OF CONTACT WITH OXYGEN. ALL AREAS OF THE STRUCTURE ARE COMPLETELY CATHODICALLY PROTECTED WHEN THE STRUCTURE EXHIBITS THE SAME POTENTIAL AS THE TEST SPECIMEN, OR IS NEGATIVE WITH RESPECT TO THE SPECIMEN.

March 1972 G. .A. MARSH ETA!- METHOD FOR DETERMINING THE COMPLETENESS OFCATHODIC PROTECTION OF CORRODIBLE METAL STRUCTURE Filed Sept. 21, 1970INVENTOR5 64 51v ,4, MARS/7 [QM/4R0 SCH/45661 BY m M ATTORNEY UnitedStates Patent @flice- METHOD FOR DETERMINING THE COMPLETE- NESS FCATHODIC PROTECTION OF CORROD- IBLE METAL STRUCTURE Glenn A. Marsh andEdward Schaschl, Fullerton, Califl, assignors to Union Oil Company ofCalifornia, Los Angeles, Calif.

Continuation-in-part of application Ser. No. 557,492, June 14, 1966,which is a continuation-in-part of application Ser. No. 213,171, July30, 1962. This application Sept. 21, 1970, Ser. No. 73,931

Int. Cl. C23f 13/00; G01n 27/46 US. Cl. 204-148 8 Claims ABSTRACT OF THEDISCLOSURE A method for evaluating the effectiveness of a cathodicprotection system in which the completeness of cathodic protection of acorrodible metal structure exposed to a corrosive environment isascertained by measuring the polarity of the structure with respect to aspecimen of the same metal placed in the corrosive environment andelectrically connected to the structure and maintained in electrolyticcontact with the corrosive environment and out of contact with oxygen.All areas of the structure are completely cathodically protected whenthe structure exhibits the same potential as the test specimen, or isnegative with respect to the specimen.

This is a continuation-in-part of application Ser. No. 557,492, filedJune 14, 1966, now U.S. Pat. No. 3,549,993, which in turn is acontinuation-in-part of application Ser. No. 213,171, filed July 30,1962, and now abandoned.

This invention relates to the cathodic protection of corrodible metalstructures, and more particularly concerns a method for determining thecompleteness of cathodic protection of such structures.

Almost any metallic surface exposed to a corrosive electrolyticenvironment, such as those disposed in damp soil or water, can becathodically protected from corrosion. In conventional cathodicprotection systems for mitigating the corrosion of submarine orsubterranean metallic structures, cathodic protection is attained byconnecting sacrificial anodes of a metal higher in the electromotiveseries than the structure, such as magnesium or zinc in the case offerrous structures, to the structure and disposing them within theelectrolytic environment. Also, direct current electricity can besupplied to the structure to provide all or part of the current requiredfor cathodic protection. Less expensive anodic metals, such as scrapiron, can also be utilized as anodes where an auxiliary current sourceis used to drive current from the anode to the structure beingprotected. Cathodic protection of the structure is achieved whencathodic areas of the structure receive all electrons utilized in thecathodic process from the auxiliary anode, and not from the local anodesof the structure itself. Generally, for carbon steel, protection isachieved when the potential of the structure has been depressed to O.85volt with reference to a saturated copper-copper sulfate half cell.Under the condition of complete cathodic protection, local cathodes inthe structure are assumed to be polarized to the open circuit potentialof the anodes.

Our prior applications Ser. Nos. 557,492 and 213,171 disclose a methodfor measuring the maximum corrosion rate of a corrodible metal structureexposed to a corrosive environment in which the maximum corrosion rateof the most anodic portion of the structure is determined by measuringthe corrosion rate of a corrosion test probe placed in the corrosiveenvironment, electrically connected to the structure and maintained inelectrolytic con- 3,649,492 Patented Mar. 14, 1972 tact with thecorrosive environment and out of contact with oxygen. Oxygen is excludedfrom contact with the corrosion probe by placing a special barrier of anelectrolytically conductive, oxygen-impermeable material around theprobe. While this technique has proved advantageous in determining themaximum corrosion rate that might be expected in a cathodicallyprotected system and, hence, can also be used to evaluate theeffectiveness of the cathodic protection system, an expensive andcomplicated Wheatstone bridge type of instrument is required to measurethe current flow through the corrosion probe in determining corrosionrate by the resistance method. Thus, need exists for a simple techniquefor evaluating the eifectiveness of a cathodic protection system thatemploys simple, inexpensive, readily available measuring instruments.

Accordingly, a primary object of the invention is to provide a methodfor evaluating the effectiveness of a cathodic protection system.

Another object of the invention is to provide a method for ascertainingand continuously monitoring, if desired, the elfectiveness of a cathodicprotection system based on a single, readily obtainable measurement.

Still another object of the invention is to provide a method fordetermining the effectiveness of a cathodic protection system employinga simple, inexpensive, readily available instrument.

A yet further object of the invention is to provide a method forcontrolling the amount of current supplied to a cathodically protectedcorrodible metal structure to attain complete cathodic protection.

These and other objects of the invention will be apparent from thefollowing description and the appended drawing which schematicallyillustrates the practice of the invention.

Briefly, the method of this invention is practiced by placing a specimenof the corrodible metal in the corrosive electrolytic environment inwhich a cathodically protected metallic structure is buried orsubmerged. The corrodible metal specimen and the structure beingcathodically protected are electrically connected by an electricconductor, and a special barrier of electrolytically conductive,oxygen-impermeable material is placed around the specimen to maintainthe specimen in electrolytic contact with the corrosive environment andout of contact with oxygen. The polarity of the structure with respectto the test specimen is then determined. All areas of the structure arecompletely cathodically protected when the structure exhibits the samepotential as the test specimen, or is negative with respect to thespecimen.

In the typical case, the corrosion rate of buried or submerged steelstructures is determined by the availability of dissolved oxygen and bythe conductivity of the electrolyte. Steel corrodes in an aqueousoxygen-containing electrolyte by local cell action, by long cell action,and by a combination of local and long cell action. Local cell actionresults in rather uniform loss of metal from the entire exposed surface.Long cell corrosion causes severe pitting of the metal surface inlocalized areas. In local cell action, available oxygen is reduced atmicroscopic cathodic sites and iron is oxidized at adjacent microscopicanodic sites. The pH of the environment is not a rate determining factorin the range from 5 to 9. As the conductivity of the electrolyteincreases, long cell action becomes more and more possible. In long cellaction, the cathodic and anodic sites or zones are separated by amacroscopic distance which may be from inch or less to more than feet,depending on the conductivity of the electrolyte. The driving force forthe long cell action is derived solely from the difference inavailability of oxygen between the two zones and is independent of themeans by which this difference is achieved. Furthermore, long cellcorrosion is a self perpetuating mechanism. Once cor-- rosion starts,corrosion products are formed at the anode which tend to occlude oxygenfrom this area, thus causing a greater diiferential concentration ofoxygen, which further accelerates the corrosion reaction.

Buried or submerged structures in damp, aerated environments are subjectto rapid localized corrosion due to differential aeration cell attack.It has been found that the oxygen which sustains galvanic corrosion canfind its way to the cathodic area of submerged structures by dissolvingin the body of water in which the structure is submerged. Similarly, airis carried to buried structures by dissolving in rain water which drainsdownward from the surface of the earth to the buried structure. Thus, inany case, the oxygen necessary to sustain the corrosion reaction isdissolved in the electrolyte solution.

Referring now to the drawing, the numeral represents a corrodible metalstructure such as a pipeline buried in earth formation 12. While thedrawing illustrates a pipe line buried in an earth formation, it is tobe understood that the shape or utility of the corrodible metalstructure is not part of the invention and that the method of theinvention can be practiced on all manner of structures. Further, it isto be recognized that structure 10 can be submerged in a body of waterrather than the illustrated earth formation. The cathodic protectionsystem for metallic structure 10 is comprised of an external D.C. powersource such as battery 14 connected to the structure through variableresistor 16 by conductor 18. Ammeter 20 measures the current flowingfrom battery 14 to structure 10. The positive terminal of battery 14 isconnected to anode 22 by conductor 24. Alternatively, anode 22 can be asacrificial anode, in which case battery 14 is not used. Sacrificialanode 22 is constructed of magnesium or other metal higher in theelectromotive series than ferrous pipeline 10. Variable resistor 16 isadjusted to control the flow of electrical current to pipeline 10 andthe amount of electrical current flowing to the pipeline is measured byammeter 20.

Specimen 33 is a small piece or coupon of a corrodible metal of the sameor similar type as corrodible structure 10. For example, wherecorrodible structure 10 is constructed of a carbon steel, specimen 33 isformed from the same or similar type of carbon steel. Specimen 33 isburied or submerged in the corrosive environment, such as subterraneanformation 12, and is electrically connected to the corrodible structureby means of electrical conductors 34 and 36 which are convenientlyconnected in junction box 38 located above the surface of earthformation 12. Normally, conductors 34 and 36 are connected in thejunction box so that specimen 33 is connected to structure 10. However,when a reading is to be made, this connection is broken and polarityindicating device 40 is connected between conductors 34 and 36. Polarityindicating device 40 can be a microammeter, a galvanometer, azero-resistance ammeter or the like. In one preferred embodiment,polarity indicating device 40 is a zero-center galvanometer having a redzone in one polarity and a green zone in the opposite polaritycorresponding to insufiicient cathodic protection and complete cathodicprotection, respectively. Polarity indicating device 40 may be connectedeach time it is desired to determine the adequacy of the cathodicprotection, or the device can be permanently installed as a fieldmonitor.

The barrier material used to ensheath the test specimen desirablyexhibits the properties of high electrolytic conductance andimpermeability to non-ionic dissolved gases and liquid fluids generally,and particularly to dissolved oxygen. Since the electrolytic conductanceof the material is achieved by the diffusion of ions through the barriermaterial, materials having high permeability for ions are preferred. Anumber of materials having both a high degree of electrolyticconductance and which are substantially impervious to oxygen andnon-ionic fluids are known in the electrolysis and electrodialysis arts.While an ideal suitable for the electrolytically conductive barrier issubstantially imperivous to oxygen, materials having limitedpermeabilities are satisfactory in many applications. Increased oxygenpermeability has the effect of reducing the observed corrosion rate byrendering the test specimen less anodic. However, errors introducedlay-using materials having low permeabilities to oxygen are generallysmall. Thus, in many applications, satisfactory test results areobtained by ensheathiug the probe in an electrolytically conductive,substantially oxygen-impermeable material.

Suitable electrolytically-conductive, substantially oxygen-impermeablematerials include animal tissue, various gels such as silica gel andalumina gel, sponge saturated with agar gel, and and paper dipped incollodion or agar gel, porous paper such as laboratory filter paper, andmultiple layers of cloth. Where a relatively thick barrier of a moistureabsorbing material such as animal tissue, agar-agar, silica gel, etc.,is used, it is preferred to incorporate therein a small amount of'ahighly ionizing salt, such as sodium chloride or sodium sulfate, toincrease the electrolytic conductivity of the barrier.

Also suitable are numerous resins which are commonly used in makingsemi-permeable barriers. Where the material utilized is of itself toosoft to provide suflicient rigidity, such as agar-agar, it may be coatedon a plastic screen, or an insulation-coated metallic screen. Thebarrier will absorb or adsorb sufficient water or electrolyte from thesurrounding earth or body of water to maintain a high electrolyticconductivity. Thus, while most barrier materials achieve an equilibriumwater content, the material is substantially impermeable to water andother nonionized fluids once the equilibrium is established.

Another barrier for isolating the apparatus of this invention from theoxygen-containing electrolytic environment while maintaining it inelectrolytic communication with the anode of the cathodic protectionsystem, is a mixture of sandy soil with about 1% to 10% by weight ofbentonite. The bentonite is capable of absorbing and retaining a smallamount of electrolyte, thereby main taining a high electrolyticconductivity. Upon absorbing water the bentonite swells and forms withthe sandy soil a barrier capable of resisting the influx of additionalground water. Thus, the barrier material Will in itself be damp or evenwet, but will be impermeable to the flow of additional quantities of theelectrolytic environment. Such a barrier, which, although damp, iscapable of resisting the flow of additional quantities of water meetsthe definition of a fluid-impermeable barrier, as used in thisspecification and in the appended claims. As in the case of some of theother barriers, it is preferred to incorporate in the bentonite a smallamount of a highly ionizing salt to enhance the electrolyticconductivity thereof.

Ferrous metal in an oxygen-free environment assumes the open circuitpotential of the anodic areas since there is no cathodic activity. Inorder to achieve cathodic protection, it is necessary to depress thepotential of the structure to the open circuit potential of the anodicareas, i.e., 0.8 5 volt with reference to a saturated coppercoppersulfate half cell. Hence, the effectiveness of the cathodic protectionof a corrodible metal structure in a corrosive environment can bedetermined by measuring the polarity of the structure and adjusting thecurrent supplied to the structure to maintain its potenial more negativethan the open circuit potential of the anodes.

In accordance with the method of this invention, the adequacy orcompleteness of the cathodic protection provided a corrodible metalstructure exposed to a corrosive electrolytic environment is determinedby placing a specimen of the same or a substantially similar metal asthe structure in the corrosive environment. The specimen is surroundedby or encased in a sheath of an electrolytically conductive,oxygen-impermeable material of the type hereinabove described so thatthe specimen is maintained in electrolytic contact with the corrosiveenvironment, but

out of contact with oxygen in the environment, i.e., oxygen in thecorrosive environment is excluded from contact with the specimen. Also,the specimen is electrically connected to the structure in theabove-described manner.

The adequacy or completeness of the cathodic protection provided thestructure is determined by connecting a microammeter, galvanometer,zero-resistance ammeter, or other current measuring device between thespecimen and the cathodically protected structure and measuring thepolarity of the structure with respect to the specimen. Since thespecimen is at its open-circuit, anodic potential, e.g., at or close to0.85 volt for carbon steel, the cathodic protection of the structure iscomplete if the structure exhibits the same potential as the specimen,in which case no current will flow between them, or is negative withrespect to the specimen. On the contrary, the cathodic protection isincomplete if the structure is positive with respect to the specimen.

Where it is ascertained that the structure has a positive potential withrespect to the specimen, i.e., there is a flow of electrons from thespecimen to the structure, complete cathodic protection can be providedby increasing the flow of current to the structure sufiiciently torender the potential of the structure equal to or more negative thanthat of the open circuit anodic potential of the specimen. Also, where acathodically protected structure is provided with excess cathodicprotection current, the current supplied can be reduced so long as thestructure is not rendered positive with respect to the specimen.

The invention is further described by the following examples which areillustrative of specific modes of practicing the invention as defined bythe appended claims.

EXAMPLE 1 The adequacy of the cathodic protection applied to a buriedpipeline is determined by the method of this invention. The pipeline isconstructed of type API 5L carbon steel, wrapped and buried andcathodically protected in conventional manner.

A small coupon of type 1020 cold rolled steel and a resistance type,temperature compensated corrosion probe are ensheathed in a mixture ofbentonite, soil and water and buried approximately 12 inches below thesurface and approximately 12 inches from the pipeline. An electricalconductor attached to the coupon and the leads from the corrosion probeextend above ground and terminate in a junction box. Also, an electricallead attached to the pipeline extends to the junction box. Both thecoupon and the corrosion test probe are maintained electricallyconnected to the pipeline except when readings are made.

A milliammeter is connected between the structure and the coupon and itis observed that the structure has a negative polarity with respect tothe coupon, i.e., an electronic current of 0.3 milliamp is observedflowing from the structure to the coupon.

EXAMPLE 2 A corrodible carbon steel buried pipeline is cathodicallyprotected substantially as illustrated in the drawings. Also, a carbonsteel coupon is encased in an electrolytically conductive, oxygenimpermeable sheath and buried near the pipeline. The coupon iselectrically connected to the pipeline through an above ground junctionbox.

The electrical leads from the pipeline and from the coupon aredisconnected and attached to a zero-center galvanometer. Thegalvanometer deflection indicates that the pipeline is positive withrespect to the coupon.

The cathodic protection current supplied to the pipeline is thenincreased and a second polarity measurement made. The pipeline is nownegative with respect to the coupon indicating that the potential of thepipeline is less than the anodic open circuit potential of the couponand, thus, that the cathodic protection is adequate to substantiallymitigate corrosion of the pipeline.

While particular embodiments of the invention have been described, itwill be understood, of course that the invention is not limited theretosince many modifications can be made and it is intended to includewithin the invention such modifications as are within the scope of theclaims.

The invention having thus been described, we claim: 1. A method fordetermining the completeness of the cathodic protection provided acathodically protected corrodible metal structure exposed to a corrosiveenvironment which comprises:

placing in said corrosive environment a specimen of a corrodible metalsimilar in composition to the corrodible metal structure under cathodicprotection;

electrically connecting said test specimen to said metal structure undercathodic protection;

maintaining said specimen in electrolytic contact with said corrosiveenvironment;

excluding any substantial quantity of oxygen from contact with said testspecimen; and

determining the polarity of the structure with respect to said specimen.

2. The method defined in claim 1 wherein said test specimen ismaintained in electrolytic contact with said corrosive environment andany substantial quantity of oxygen is excluded from contact with saidspecimen by encasing the specimen in a sheath of electrolyticallyconductive, substantially oxygen impermeable material.

3. The method defined in claim 2 wherein said electrolyticallyconductive, substantially oxygen-impermeable material is selected fromthe group consisting of animal tissue, agar-agar, silica gel, aluminagel, a mixture of sandy soil and about 1 to 10 percent by weightbentonite, a plurality of layers of cloth, and a plurality of layers ofporous paper.

4. The method defined in claim 3 wherein a small amount of a stronglyionizing salt is added to the material forming said sheath.

5. The method defined in claim 1 wherein the structure is cathodicallyprotected by supplying electrical current to said structure and whereinthe flow of electrical current to said structure is adjusted so that thestructure exhibits a potential equal to or more negative than thepotential of the specimen.

6. The method defined in claim 1 wherein the polarity of the structurewith respect to the specimen is determined by connecting a microammeter,a galvanometer, or a zero-resistance ammeter between the structure andthe specimen.

7. A method for determining the completeness of the cathodic protectionprovided a cathodically protected corrodible metal structure exposed toa corrosive environment which comprises:

placing in said corrosive environment a specimen of a corrodible metalsimilar in composition to said corrodible metal structure;

encasing said specimen in a sheath of electrolytically conductive,oxygen-impermeable material selected from the group consisting of animaltissue, agar-agar, silica gel, alumina gel, a mixture of sandy soil andabout 1 to 10 percent by weight bentonite, a plurality of layers ofcloth, and a plurality of layers of porous paper whereby said specimenis maintained in electrolytic contact with said corrosive environmentand out of contact with any substantial quantity of oxygen undercathodic protection;

electrically connecting said specimen to said corrodible metalstructure;

periodically connecting a microammeter, a galvanometer, or azero-resistance ammeter between the structure and the specimen; and

determining the polarity of the structure with respect to said specimen.

8. A method for cathodically protecting a corrodible metal structureexposed to a corrosive environment which compnses:

placing in said corrosive environment .a specimen of a corrodible metalsimilar in composition to the corrodible metal structure;

electrically connecting, said test specimen to said metal structure;

encasing said specimen in a sheath of electrically conductive,oxygen-impermeable material so that said specimen is maintained inelectrolytic contact with said corrosive environment and out of contactwith any substantial quantity of oxygen;

determining the polarity of the structure with respect to said specimen;

supplying electrical current to said metal structure to cathodicallyprotect the metal structure;

determining the polarity of said structure with respect to saidspecimen; and

adjusting the flow of current to said structure so that it exhibits apotential equal to or more negative than the potential of the specimen.

References Cited UNITED STATES PATENTS Cowles 204-195" TA-HSUNG TUNG,Primary Examiner US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF QORRECTIQN Patent No. 3 6 49+92 D d March 1 1972 Inventor(s) Glenn A. Marsh and Edward Schaschl Itis certified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 6, claim 7, line 65:

"under cathodic protection" should be deleted.

Column 6, claim 7, line 67:

"under cathodic protection" should be added before semi-colon.

Signed and sealed this 18th day of July 1 972.

(SEAL) Attest:

EDWARD M.FLETGHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents

